Circadian rhythm disturbances are associated with metabolic syndrome and obesity in humans, leading to altered homeostatic regulation of glucose, insulin, and lipids. Bile acids are physiological detergents essential for the transport and absorption of nutrients and metabolites, and also act as nuclear receptor substrates in the cell signaling pathways that modulate lipid and glucose energy metabolism and homeostasis. Transcriptional activity of cholesterol 7?-hydroxylase (CYP7A1), the rate-limiting enzyme in the conversion of cholesterol to bile acids, is strictly regulated by components of the cellular environment, including bile acids, nutrient availability, insulin, and other factors. Recent studie also indicate that CYP7A1 displays a diurnal rhythm of expression in rodents and humans, and this rhythm is shifted under conditions of altered diet and time of feeding, leading to disturbed regulation of bile acid homeostasis and metabolism. The proposed experiments will further examine the interactions of circadian rhythms and hepatic metabolism in a CYP7A1 humanized mouse model undergoing sleep deprivation and high-fat feeding to elucidate the mechanisms by which circadian misregulation contributes to Type II diabetes and obesity in humans.